Diaphragms for Use in the Electrolysis of Alkali Metal Chlorides

This invention relates to electrolytic diaphragm cells. More particularly, this invention relates to novel diaphragms for electrolytic diaphragm cells.

Production of chlorine and alkali metal hydroxides in diaphragm cells which electrolyze alkali metal chloride solutions has been a commercially important process for a number of years. The diaphragm cell employs an anode and a cathode separated by a fluid permeable diaphragm. Maintenance of the desired fluid permeability of the diaphragm is an economically desirable aspect in the operation of the diaphragm cell. While asbestos has been the primary material employed in diaphragms in commercial chlorine cells, there has been an extensive search for materials having improved cell life.

It is known to employ inorganic materials such as glass, sand, or corundum in diaphragms for electrolytic cells where they are combined with a binding agent. Inorganic binders such as hydraulic cement are cited in, for example, U.S. Pat. Nos. 512,503, issued to Craney; 579,250, issued to Baker; and 609,745, issued to Luxton. These diaphragms were found to be defective because their density and bulkiness caused large power losses. British Pat. No. 312,713, issued to Mueller, teaches the use of organic materials such as rubber or gutta percha as well as cellulose and thermoplastic cellulose esters like cellulose nitrate. Cellulose esters, however, are readily decomposed when in contact with alkali metal hydroxide solutions. These diaphragms were readily replaced by asbestos compositions in commercial cells for the electrolysis of alkali metal chloride solutions.

The use of asbestos, however, produces diaphragms of limited cell life as the asbestos fibers swell and dissolve to form gel layers during electrolysis indicating both physical and chemical alteration of the fibers. In addition, asbestos is now identified by the Environmental Protection Agency as a health hazard.

U.S. Pat. No. 1,742,411 issued to J. Mueller describes molded diaphragms having as a filling material ingredients such as quartz sand, glass sand, glass wool or wool of asbestos mixing with a binding agent which is plastic in the hot state. Mueller's binding agents include asphalt, goudron, pitch, tar and resins. These are natural products having indefinite structures and include both aliphatic and aromatic groups with diverse substituents. When used in electrolytic cells for the production of chlorine, these materials undergo chlorination reactions which are degrading and produce oils and tars which contaminate cell components and products.

Therefore there is a need for diaphragms having increased operating life while employing materials which are inexpensive.

It is an object of the present invention to provide a diaphragm having increased stability and a longer operational life when employed in the electrolysis of alkali metal chloride solutions.

Another object of the invention is the use of ecologically acceptable non-polluting materials in diaphragm compositions.

An additional object of the invention is a diaphragm having support materials which are chemically and physically stable during electrolysis.

A further object of the invention is the production of diaphragms from inexpensive materials.

Briefly, the novel diaphragm of the present invention for use in the electrolysis of alkali metal chloride brines which comprises an electrically non-conductive, non-swelling support material selected from the group consisting of fibers, meshes and fabrics impregnated with a mixture of sand and a synthetic thermoplastic polymeric binding agent.

The term "sand" includes compositions having a silicon dioxide content of at least about 95 percent by weight. Suitable sands include silica, quartz and silica sand among others.

It is desirable that the sand have a suitable particle size, for example, smaller than about 40 mesh and larger than about 300 mesh and preferably from about 100 to about 200 mesh (Tyler Standard Screen Scale).

As a binding material a synthetic thermoplastic polymeric composition is employed which is resistant to the gases and solutions which are found in a cell for the electrolysis of alkali metal chloride solutions.

Examples of suitable thermoplastic polymeric binding agents are those synthesized from derivatives of petroleum or coal and include, for example, polyarylene compounds and polyolefin compounds. These polymers comprise monomers or recurring units of defined structures which are synthesized and then polymerized by known methods.

Polyarylene compounds include polyphenylene, polynaphthylene and polyanthracene derivatives. For example, a useful group of binding agents are polyarylene sulfides such as polyphenylene sulfide or polynaphthylene sulfide. Polyarylene sulfides are well known compounds whose preparation and properties are described in the Encyclopedia of Polymer Science and Technology (Interscience Publishers) Vol. 10, pages 653-659. In addition to the parent compounds, derivatives having chloro-, fluoro- or alkyl substituents may be used such as poly(perfluorophenylene) sulfide and poly(methylphenylene) sulfide.

Polyolefin compounds suitable as binding agents include polymers of olefins having from 2 to about 6 carbon atoms in the primary chain, for example, polyethylene, polypropylene, polybutylene, polypentylene and polyhexylene, as well as their chloro- and fluoro-derivatives such as polyvinyl chloride, polyvinylidene chloride, polytetrafluoroethylene, fluorinated ethylene-propylene (FEP), polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, copolymers of ethylene-chlorotrifluoroethylene, and perfluoroalkoxy resins.

Mixtures of polyarylene compounds and polyolefin compounds may be used as binding agents. For example, polyarylene sulfides may be mixed with polyolefins such as polytetrafluoroethylene, polychlorotrifluoroethylene or polyvinylidene fluoride in any suitable proportion. Preferred proportions are those in which the polyarylene sulfide is from about 30 to about 90 percent by weight of the mixture.

Thermoplastic polymeric binding agents are used in particulate forms such as granules or powders where the particle size is preferably smaller than 100 mesh and greater than about 325 mesh, and preferably from about 150 to about 250 mesh.

In preparing the novel diaphragms of the present invention, any suitable proportions of sand and the thermoplastic polymeric binding agent may be employed which provide the desired permeability. For example, mixtures comprising from about 40 to about 90 percent by volume of sand and from about 60 to about 10 percent by volume of thermoplastic polymeric binder may be employed. Preferably, the diaphragms comprise mixtures of from about 50 to about 70 percent by volume of sand and from about 50 to about 30 percent by volume of thermoplastic polymeric binding agent.

The sand and polymeric organic binder are blended as dry particles or in slurry form by known methods to produce a substantially homogeneous mixture.

It may be desirable to employ additives such as lubricants or wetting agents in the mixture.

Examples of lubricants include granular materials having a melting point above about 100.degree. C. such as graphite, zinc stearate, calcium stearate, stearic acid, and synthetic amide waxes which are used in amounts of from about 0.25 to about 10 percent by volume of the total mixture of sand and binding agent. Where a conductive material such as graphite is added, the amount used is insufficient to make the diaphragm electrically conductive.

Suitable wetting agents include surface active agents such as alkyl aryl polyether alcohols which are used in any suitable amounts, for example, from about 0.5 percent to about 3 percent by volume of the mixture.

If desired, the mixture may contain other additives such as alumina, inorganic phosphates, lithium salts, lime, magnesia or inorganic magnesium compounds to provide improved ionic conductivity and cation exchange properties. Preferred additives are magnesia or inorganic magnesium compounds. These additives may be used in amounts of from about 5 to about 20 percent by volume of the mixture.

Diaphragms of the present invention are formed by melt processing the mixture, for example, by heating at temperatures up to about 350.degree. C. for a short period of time and cooling to form a cohesive shaped body having a permeability suitable for use in the electrolysis of alkali metal chlorides.

Where added mechanical support is desired, materials in the form of fibers, meshes or fabrics may be incorporated in the mixture. The support materials should be those which can be suitably used in wet filtration processes without swelling or dissolving in the electrolyte. In addition to being chemically resistant to and dimensionally stable in the gases and liquids present in the electrolytic cell, the materials should have a suitable permeability to gases such as air. Suitable support materials are those which are non-conductive such as glass wool or synthetic organic thermoplastic polymers; or conductive including steel wool and meshes of nickel, steel, or titanium. In forming diaphragms containing conductive materials as mechanical support, care is taken to encapsulate the conductive material in the mixture to prevent the diaphragm from becoming electrically conductive. The support materials should be sufficiently flexible so that they can be deposited on or shaped to various electrode structures. Preferred forms of the support material are a felt fabric or staple fibers. When employed as the support material, staple fibers constitute a weight percent of the total mixture of from about 5 to about 20 percent. In one embodiment, suitable support materials include fibers, meshes or fabrics of plastic materials such as polyolefins which are polymers of olefins having from about 2 to about 6 carbon atoms in the primary chain as well as their chloro- and fluoro-derivatives.

Examples include polyethylene, polypropylene, polybutylene, polypentylene, polyhexylene, polyvinyl chloride, polyvinylidene chloride, polytetrafluoroethylene, fluorinated ethylene-propylene (FEP), polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, and copolymers of ethylene-chlorotrifluoroethylene.

Also suitable support materials are polyaromatic compounds such as the polyarylene compounds discussed above as binding agents.

The support materials may be impregnated with the sand and binder material in any of several ways. For example, a slurry of sand and the binder, in a solution such as cell liquor, is prepared and the support material is impregnated by soaking in the slurry. Another method is to attach the supporting material to the cathode and immerse the cathode in the slurry, using suction means to draw the slurry through the support material.

Following impregnation with the sand and the binding agent, the diaphragm is dried and then heated to form a cohesive body having a suitable permeability. Diaphragms, for example, having air flow values of from about 0.1 to about 60, preferably of from about 1 to about 30 cubic feet per minute per square foot of diaphragm. Air flow values for the diaphragm may be determined, using, for example, American Society for Testing Materials Method D737-75, Standard Test Method for Air Permeability of Textile Fabrics.

Uniform permeability throughout the diaphragm is not required and it may be advantageous to have a greater permeability for the portion of the diaphragm which will be positioned closest to the anode than in the portion nearest the cathode.

The diaphragms of the present invention have handling properties which far exceed those of, for example, asbestos. The supported diaphragms can be removed from the cell, washed or treated to restore flowability, and replaced in the cell without physical damage. During operation of the cell, the novel diaphragms remain dimensionally stable with the support material neither swelling nor being dissolved or deteriorated by the electrolyte or the cell products produced.

Diaphragms of the present invention have an extended service life with little evidence of loss of flow properties due to plugging. The diaphragms are produced from non-polluting inexpensive materials using economical methods of production.

Electrolytic cells in which the diaphragms of the present invention may be used are those which are employed commercially in the production of chlorine and alkali metal hydroxides. The cells have an anode assembly containing a plurality of foraminous metal anodes, a cathode assembly having a plurality of foraminous metal cathodes with the novel diaphragm separating the anodes from the cathodes. Suitable electrolytic cells include, for example, those types illustrated by U.S. Pat. Nos. 1,862,244; 2,370,087; 2,987,463; 3,247,090; 3,477,938; 3,493,487; 3,617,461; and 3,642,604.

The permeable diaphragms of the present invention are illustrated by the following examples without any intention of being limited thereby.

Sand (99 percent SiO.sub.2), having a particle size smaller than 100 mesh, was added to a tumbler along with polyphenylene sulfide resin (Phillips Petroleum Company, Ryton-PPS type V-1, a polyphenylene sulfide resin) particles smaller than 200 mesh and graphite having a particle size of less than 100 mesh. The components were blended for about two hours to provide a mixture containing (by volume) 50 percent sand, 40 percent resin and 10 percent graphite. The mixture was poured into a mold and heated to a temperature of 330.degree. C. Pressure was then applied (12 kg/cm.sup.2) and the mixture allowed to cool down under pressure. Into an electrolytic cell containing brine having a sodium chloride concentration of 315-320 grams per liter, the porous shaped diaphragm (3.5.times.5.5 inches) was placed adjacent to the cathode. Electrolysis of the brine was conducted at a current density of 2 KA/m.sup.2 period of 20 days to produce Cl.sub.2 gas and sodium hydroxide at a concentration of 115-170 grams per liter at an average power consumption in the range of 2250-2700 kilowatt hours per ton of Cl.sub.2. During the period of operation, no evidence of plugging was found.

A homogeneous mixture was prepared containing 50 percent by volume of sand (99 percent SiO.sub.2); 40 percent by volume of a resinous mixture of polyphenylene sulfide and polytetrafluoroethylene (available from Liquid Nitrogen Products Company under the trade name 2002-PPS); and 10 percent by volume of graphite. All components had a particle size of 100 mesh or less. Following blending, the mixture was placed in a mold along with a nickel mesh used as a support material (Exmet Corp. Distex brick 5 Ni 35-1/0) and heated in an oven to 350.degree.-400.degree. C. for about 30 minutes. After removal from the oven, a pressure of 12 kg/cm.sup.2 was applied to the mold during the cooling period. The prepared diaphragm, 2-3 mm thick, was positioned adjacent to the cathode in a cell for the electrolysis of sodium chloride brines containing 315-320 grams per liter of NaCl. Brine at a temperature of 85.degree.-90.degree. C., was electrolyzed at a current density of 2 KA/m.sup.2 of anode surface to produce chlorine gas and sodium hydroxide at a concentration of 135-170 grams per liter of NaOH and containing 160-190 grams of NaCl. The cell was operated for 130 days, with an average power consumption in the range of 2300-2460 KWH/ECU. During the period of operation, with the anolyte head level maintained at about 2 inches, there has been no evidence of pluggage of the diaphragm.

A diaphragm of the type of Example 2 was produced and placed in a mold. A layer of the mixture of sand and polyphenylene sulfide used in Example 1, was placed on top of the diaphragm and the mold heated in an oven at 350.degree.-400.degree. C. for about 1/2 hour. After removal from the oven, pressure was applied during the cooling period and a layered diaphragm produced. The layered diaphragm was installed in a cell for the electrolysis of brine containing 315-320 grams per liter of NaCl and the head level maintained at about 3 inches. The diaphragm was positioned in the cell such that the top layer of sand and polyphenylene sulfide faced the anolyte. The cell was operated for 100 days at a current density of 2 KA/m.sup.2. Chlorine gas and caustic soda (140-165 grams per liter NaOH) were produced at a power consumption in the range of 2400-2600 KWH/ECU. No evidence of diaphragm plugging was found during cell operation.

An aqueous slurry of polyphenylene sulfide resin (particle size smaller than 200 mesh) containing an octylphenoxy polyethoxy ethanol wetting agent (Rohm & Haas Triton X-100) was poured into a blade mixer. To the mixer was added sand, and graphite particles (smaller than 100 mesh) and the components mixed for about one hour. The slurry, containing (by volume) 50% sand, 40% polyphenylene sulfide, 9% graphite and 1% wetting agent was poured into a mold and let dry under natural convection. The mold was baked at 330.degree. C. for about 1/2 hour and a diaphragm in the form of a cohesive shaped body produced.

Silica sand, having particle sizes in the range between 100 mesh and 300 mesh, was mixed with a resinous mixture of polyphenylene sulfide and polytetrafluoroethylene (2002-PPS), and a mineral product having a particle size of less than 300 mesh containing about 20 percent of magnesia (IGS sold by Industrial Mineral Ventures of Golden, Colorado). A volume ratio of 54 percent silica sand, 6 percent mineral product, and 40 percent resinous binding agent was used. The solids were fed to a tumbler to provide a uniform mixture. The solid mixture was added to cell liquor containing 12 percent NaOH and 15 percent NaCl to form a slurry.

A section of polytetrafluoroethylene fiber felt (DuPont's Armalon XT-7550) having a thickness of 0.125 inch was immersed in the slurry and mixed for several hours in the slurry. After mixing it was allowed to remain in the slurry for several days. The surface area of the section was about 20 percent larger than that of the cathode with which it would be used. After removing from the slurry, the felt diaphragm was dried and then heated for about 0.5 hours at 350.degree. C. The diaphragm had an air permeability in the range of 0.1 to 1 cubic feet per minute.

The impregnated felt diaphragm was placed adjacent to a foraminous steel cathode in an electrolytic cell into which sodium chloride brine at a concentration of 315-320 grams per liter of NaCl was fed. Using a ruthenium oxide coated titanium mesh anode, current was passed through the brine at a density of 2.0 KA/m.sup.2.

Caustic soda at a concentration range of 170 to 210 grams per liter of NaOH was obtained during the three weeks the cell was continuously operated. The cell voltage range was from 3.31 to 3.56 volts with the cell being operated at a brine temperature of 90.degree. C. Average power consumption per ton of Cl.sub.2 produced was in the range of 2410-2700 kilowatt hours with a cathode current efficiency in the range of 89-98 percent.

During cell operation the support material remained physically and chemically stable with no signs of swelling or deterioration.

Into a slurry identical to that employed in Example 5 was added polytetrafluoroethylene staples (6.67 denier) having a length of 0.5-1 inch. Staples added to the slurry amounted to about 10 percent by weight of the total weight of the slurry. After curing, a steel screen cathode was immersed in the slurry. Following agitation of the slurry by air, a diaphragm was deposited on the cathode by applying a vacuum from the cathode side until a vacuum in the range of 23-27 inches was obtained. The cathode remained under vacuum for a few minutes and was then removed from the slurry, dried and the deposited diaphragm baked as in Example 5. The diaphragm had an air permeability in the range of 1-5 cubic feet per minute.

The cathode was installed in the cell employed in Example 5, the cell being operated continuously for two weeks. During this period, sodium chloride brine (315-320 grams per liter of NaCl) was electrolyzed at cell voltages in the range of 3.2-3.6 volts to produce a catholyte liquor having a NaOH concentration of 135-178 grams per liter. Operating temperature of the cell was in the range of 85.degree.-95.degree. C. At a current density of 2.0 KA/m.sup.2, power consumption was 2320-2350 kilowatt hours per electrolytic chlorine unit. Power consumption was 2700 KWH/ECU at a current density of 2.5 KA/m.sup.2.